1. Field of the Invention
The present invention relates to temperature control systems for cookware and, particularly, to a novel thermal limiting system and method for controlling application of thermal energy to a burner element of a cookware apparatus.
2. Discussion of the Prior Art
The life of the glass ceramic material forming a cooking surface or burner in a cookware apparatus is dependent on the temperature it is subjected to. Therefore, the power to a burner must be limited to prevent premature failure of the glass. The temperature of the glass is a function of time, burner power and the properties of the cooking utensil place on it (e.g. flatness, reflectivity, contents, etc.) consequently a method of dynamically adjusting the power to prevent overheating is needed, i.e. thermal limiting control.
In conventional systems, the temperature is limited in two ways: 1) by using of a temperature switch that interrupts power to the burner at excessive temperatures such as described in U.S. Pat. No. 6,150,641, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein; or, 2) by directly sensing the temperature and applying appropriate feedback control such as described in U.S. Pat. No. 6,285,012, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein.
The first thermal limiting approach 10, as described in U.S. Pat. No. 6,150,641, and illustrated in FIG. 1(a), includes implementing a thermal switch and bang-bang thermal limiting to control the temperature 18 of the cookware burner 12, and incorporates a power control component 14 receiving the power command signal 16 which, in this approach, constitutes the user power command signal. This approach is inexpensive but results in large swings in power and temperature of the cooking utensil. That is, in this first approach, a thermal switch is used to provide bang-bang temperature control when the temperature exceeds the predetermined limit. This type of control results in the frequent cycling of the power causing corresponding swings in the pan temperature.
FIG. 2(a) illustrates an example simulation of bang-bang thermal control implemented for a ceramic burner. In the example simulation, the thermal switch is modeled as a relay with an arbitrary 30xc2x0 C. of hysteresis, and the thermal response of the burner (e.g., glass temperature output) is modeled as a first order linear model (derived empirically). Initially, as shown in FIG. 2(a), the user-demanded power setting (user power command signal) is about one-half (50%)of the maximum power. At this initial setting, thermal limiting does not engage as indicated in FIG. 2(b). At the time indicated at 141, the user increases the power to 100% (FIG. 2(a)) causing the conduction state 145 of the thermal switch (e.g., bi-metallic switch) to change in accordance with bang-bang thermal limiting at time indicated as time 142. In FIG. 2(b), the conduction on/off states, i.e., engagement of bang-bang thermal limiting, is represented as the plot 145. At this setting, the glass temperature of the burner increases to the thermal limit 182, e.g., the safety thermal limit of a glass burner, as shown in FIG. 2(c). Finally, the user reduces the power back to its initial one-half power level and thermal limiting ceases, as indicated at time 143 in FIG. 2(a).
The second thermal limiting approach 20, as described in U.S. Pat. No. 6,285,012, and illustrated in FIG. 1(b), includes implementing a thermal limiting controller component 22 that limits thermal heating of burner 12xe2x80x2 in accordance with the user power command signal 16xe2x80x2, a predetermined thermal limit signal 25, and an instantaneous sensed temperature 28 that is feedback from a temperature sensor element included with the burner 12xe2x80x2. As described in U.S. Pat. No. 6,285,012, the controller includes proportional plus integral control, minimum selector and anti wind-up control elements (not shown) to provide thermal limiting for a burner 12xe2x80x2 implementing a sensor. The output 15 of the thermal limit controller 22 is input to a further power control unit for adjusting, e.g., quantizing the thermal limiter power output. This approach provides for very smooth power and temperature profiles but the temperature sensor is often expensive.
It would thus be highly desirable to provide a thermal limiting system and method for providing thermal limiting control to a cooktop burner of an electric cooking device, that provides for very smooth power without the use of an expensive thermal sensor.
A system and method for smoothly limiting the temperature of a burner of a cooking appliance, e.g. a stove ceramic burner, without the use of a temperature sensor. The method includes the steps of sensing the conduction state of a thermal switch in a bang-bang thermal limiting burner, and feeding back a signal representing this switch conduction state to control duty-cycle (and thus xe2x80x9conxe2x80x9d time) of the applied power. The power to the burner is reduced until the sensed duty-cycle cycling is reduced (lower frequency and amplitude) resulting in smoother power and temperature control.
Preferably, this sensed duty-cycle cycling is increased to near 100%, i.e., the thermal switch conducting state is almost always on, i.e., off-time is reduced.